Pellicular laminate means for shielding structures from electromagnetic radiation

ABSTRACT

A pellicular laminate comprising: a web of polypropylene film, on at least one surface of which is a layer of polyurethane; a metal, preferably a ductile metal such as gold, which has been deposited on that surface of the polyurethane layer farthest away from said web, by sputtering; a layer of adhesive on the exposed metal surface; and a release liner in the form of a film of polyester or the like covering, but separable from, said adhesive layer. With the web and the release liner removed, the remaining pellicular laminate is useful in shielding structures from electromagnetic radiation by applying the laminate to the surface of the structure to be shielded with the adhesive side towards the structure.

This is a division, of application Ser. No. 609,474, filed May 11, 1984.

BACKGROUND OF THE INVENTION

The shielding of structures, such as aircraft, instrumentation andcomponents thereof, enclosed in a housing, and the like, fromelectromagnetic radiation emanating from within or from outside thestructure is a desideratum for such purposes as preventing detection ofthe aircraft or instruments enclosed in the housing, by means whichdepend on detection of electromagnetic radiation, and for protecting theaircraft and instrumentation within the aircraft from malfunctioning asa result of such electromagnetic radiation. Because of the ineveitablepresence of cracks, fissures, seams, and other openings on the surfaceof aircraft and/or housings, and the like, containing instrumentation,effective and practical shielding of electromagnetic radiation of suchhas been difficult to achieve. While one solution to the problem wouldappear to be to apply a protective layer or skin of some kind,containing a barrier to electromagnetic radiation, over the structure tocover such cracks, fissures, seams, and openings, no completelysatisfactory means has yet been effected for accomplishing this goal.Thus, a part but only a part, of the problem is that any coating appliedto, for example, an aircraft, has to effectively resist abrasion andother destructive forces placed on the skin of the aircraft as it movesthrough the atmosphere. Furthermore, expansion and contraction of theairplane skin causes separation of coating from the aircraft skin. Suchis further aggravated by temperature changes. In short, there is a realneed for an effective practical means for achieving adequateelectromagnetic shielding which remains effective during varyingconditions of weather, abrasive forces, temperature gradients overvarious parts of the structure resulting in associated variableexpansion and contraction of surfaces such as the skin of an aircraft orinstrument housings.

SUMMARY OF THE INVENTION

We have now discovered an effective and practical means for shieldingstructures as for example, aircraft, from electromagnetic radiation, byapplying to the surface or surfaces of the structure, a uniquepellicular laminate. Our novel laminate when applied to a structure inaccordance with our invention effects adequate shielding of the entirestructure, and contents thereof, from electromagnetic radiation whichwould otherwise move into or out of the structure. Our novel pellicularlaminate can be readily prepared, stored, transported, and quicklyapplied to the structure which is to be shielded without requiringcomplicated or expensive application equipment. Our invention will bemore readily understood in light of the following detailed descriptiontogether with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view in cross section illustrating the layers making up alaminate, which is rolled up into roll "A" for subsequent lamination toroll "B".

FIG. 2 is a view in cross section showing layers making up anotherlaminate, which is rolled up into roll "B" for subsequent lamination toroll "A".

FIG. 3 is a view of rolls "A" and "B" and the method by which thelaminates on these two rolls are pressed together to form the pellicularlaminate shown in FIG. 4.

FIG. 5 is a view in cross section, of the applicants' novel pellicularlaminate after it has been applied to a surface of an object which is tobe shielded against electromagnetic radiation.

The laminates shown in FIGS. 1, 2 and 4 are laminates which are preparedfor use in forming the final, or ultimate, pellicular laminate that isillustrated in FIG. 5. This latter is the pellicular laminate which isadhesively secured to the surface of the structure being shielded.

DESCRIPTION OF THE INVENTION

In producing our novel pellicular laminate, a coating, or film, ofpolyurethane is first applied, as by casting, onto a carrier web inaccordance with procedures well known in the art. Besides casting, whichis preferred, the coating or film of polyurethane can be applied ontothe carrier web by spraying, doctoring, brushing, or the like, of asolution of polyurethane in a suitable solvent. One commercial form ofsuch polyurethane is that having the tradename of ESTANE 5707,manufactured by the B. F. Goodrich Company of 6100 Oak Tree Boulevard,Columbus, Ohio. It is preferably cast onto the carrier web in the formof a solution of the polyurethane resin in tetrahydrofuran, at asuitable concentration, and amount, so that on evaporation of thesolvent the film thickness of the resulting polyurethane film will be,preferably, from about 0.5 to about 1.5 mils. The carrier web can be ofany suitable thickness, but we have found that a 4-5 mil. thickness ofpolyethylene, polypropylene, or Teflon film is quite adequate.Preferably the carrier is a polypropylene film such as Treafilm,available from TREA Industries of North Kings Town, Rhode Island. Theresulting composite, or laminate which can be conveniently 60 incheswide and in the form of a roll containing 3000 feet of product, is thenrun through a vacuum metallizing sputter coater, and a deposit of metal,preferably a ductile metal, such as pure gold, 100 to 1000 Angstromsthick, is placed on the polyurethane surface. It is highly desirablethat the deposition of gold take place at a relative low temperature,for example, less than about 150° F. for best results. The technique andprocedure of effecting such sputter deposition is otherwise well known,and described, for example, in U.S. Pat. Nos. 4,322,276 and 4,426,275.We wish to point out that a characterizing feature of sputteringprocesses is that almost any metal can be deposited on the target, suchas a film, regrdless of the melting point of the metal or the complexityof the alloy or mixture being applied.

After passing through the sputter coater, this laminate is wound up ontoa roll, shown as roll "A" in FIG. 3.

A second laminate is prepared by casting a layer of adhesive, from about0.0001 to 0.001 mils. in thickness, onto a silicone-treated polyesterfilm, suitably 1 to 2 mils. thick, also referred to hereinafter as arelease liner. A suitable adhesive is the pressure-sensitive,self-curing, film-forming acrylic solution, sold under the tradename ofDURO-TAK 80-1057, by the National Starch and Chemical Corporation ofBridgewater, N.J. A typical silicone coated polyester film which can beused is a product from Custom Coating and Laminating, Inc., 717Plantation Street, Worester, Massachusetts. This laminate is shown inthe drawings as roll "B".

The aforesaid two laminates are then run through a laminator ofconventional design, and a third laminate is formed, as shown in FIG. 3,and rolled up to form a roll "C".

In a preferred form, the laminate on roll "C" will be composed of thefollowing layers:

4-5 mil polypropylene carrier web;

0.25 to 1.5 mil. cast polyurethane;

100 to 1 K Angstroms of sputtered metal;

A 0.0001 to 0.001 mil thick layer of adhesive;

A 1 to 2 mil. silicone treated release liner (polyester).

In use the silicone-treated release liner is peeled away from theremainder of the composite, and the adhesive side of the remainingcomposite is applied to the structure which is to be shielded. Afterapplication the polypropylene web is removed, leaving a three-layercomposite of cast polyurethane, gold, and pressure-sensitive adhesive.It is to be understood of course that the composite applied to thestructure to be shielded can be in any desired width or length, and willdepend on the size and configuration of the structure being covered.

Besides the radiation-shielding characteristics of our pellicularlaminate, a further feature stems from the fact that as the surface ofthe structure which has been coated contracts and expands as a result ofmechanical, aerodynamic, and temperature variations, our novel laminatecontracts and expands accordingly. It can be stretched substantially, toas much as 20 percent, at which point, although the resistivity goes up,the material remains conductive, while on release it goes back to itsoriginal value, functioning almost like a variable resistor.

Besides all of the above, an additional important feature of ourinvention is that the pellicular laminate which has been applied to thestructure being shielded from electromagnetic radiation, can be appliedin the form of strips or sheets of varying size and configuration, andbecause of its flexibility can be readily applied to irregular surfaces,i.e., those having protuberances and indentations. Additionally, thestrips or sheets, in whole or in part, can be quickly and easily removedfrom the surface to which they had been attached, and quickly easilyreplaced, in whole or in part, as desired, by replacement sheets, orstrips, of our pellicular laminate.

In the foregoing specification we have described preferred forms of ourinvention in their application to the shielding of such structures asaircraft and instrument housings, from electromagnetic radiation.However those skilled in the art will appreciate that our invention maybe modified in various ways, only some of which may have beenspecifically mentioned herein and may be applied to different types ofstructures, all without departing from the spirit and scope of ourinvention. Therefore it is to be understood that our invention is notlimited to the specific forms described herein or in any manner otherthan by the scope of appended claims when given the range of equivalentsto which our invention may be entitled.

What is claimed as the invention is:
 1. A pellicular laminate comprisinga carrier web of a polyolefin; a continuous coating of polyurethane on asurface of said web; a conductive layer consisting essentially of metalsputter-deposited on the exposed surface of the polyurethane coating; acoating of pressure-sensitive adhesive on said metal surface; and asilicone-coated film covering said adhesive layer.
 2. The pellicularlaminate of claim 1 wherein said carrier web is formed of polypropylene.3. The pellicular laminate of claim 1 wherein said carrier web is formedof polyethylene.
 4. The pellicular laminate of claim 1 wherein saidmetal is a ductile metal.
 5. The pellicular laminate of claim 1 whereinsaid metal is gold.
 6. The pellicular laminate of claim 1 wherein saidfilm is formed of polyester.
 7. The pellicular laminate of claim 1wherein said polyurethane coating is from about 0.25 to 5 mils. inthickness.
 8. The pellicular laminate of claim 1 wherein the saidpolyurethane is cast on the carrier web.
 9. The pellicular lamminate ofclaim 1 wherein the carrier web is formed of Teflon.
 10. The pellicularlaminate of claim 1 having a surface resistivity of less than 5 ohms persquare.
 11. The pellicular laminate of claim 5 wherein the thickness ofthe gold is from 100 to 1000 Angstroms.